Dissolution and Precipitation of Solids

As mentioned earlier, the composition of natural groundwaters depends on the composition of the geological formations where they originate from they contain dissolved rock and soil components that were soluble under the conditions (such as temperature and pressure) of their formation. Their dissolution is governed by the law of thermodynamics that is, dissolution occurs when the solution is undersaturated with respect to components such as rocks and soils. Provided that the solid components are present in sufficient quantity and there is no kinetic barrier, this process may lead to a thermodynamic equilibrium. The reversed process of dissolution is precipitation, that is, the formation of a solid phase from the dissolved components of a supersaturated solution. The composition of the 

The weathering of minerals can be understood as a continuous dissolution-precipitation process involving them. The process can be very complicated, leading to multiphase equilibria, in which more than one solid phase, solution, and gas phase may be present. For example, primary silicates transform to secondary silicate minerals via such weathering reactions (Stumm and Wollast, 1990), as in the formation of kaolinite (AljSijC OH) from anorthite (CaAl2Si208)  

The ions produced through the weathering of silicates can be classified into three groups  

Alkali metal ions (sodium and potassium ions) that are usually dissolved in groundwater. 

Silicon, aluminum, and iron(III) that form oxide, hydroxide, colloids. The proportion of their aqua complexes is extremely low, and they transform to crystalline aluminosilicates. 

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It is necessary to consider a number of equilibrium reactions in an analysis of a hydrometallurgical process. These include complexing reactions that occur in solution as well as solubility reactions that define equilibria for the dissolution and precipitation of solid phases. As an example, in analyzing the precipitation of iron compounds from sulfuric acid leach solutions, McAndrew, et al. (11) consider up to 32 hydroxyl and sulfate complexing reactions and 13 precipitation reactions. Within a restricted pH range only a few of these equilibria are relevant and need to be considered. Nevertheless, equilibrium constants for the relevant reactions must be known. Furthermore, since most processes operate at elevated temperatures, it is essential that these parameters be known over a range of temperatures. The availability of this information is discussed below. 

In this chapter we give a brief overview of the structure and composition of the Earth, discuss some properties of rocks and minerals, explain the origin of minerals and the consequences of weathering, analyze the dissolution and precipitation of solids, and present key soil phenomena and components. 

The most important leachate reactions are dissolution and precipitation of solids and minerals, redox (reduction/oxidation) reactions with organic matter, and ion exchange and sorption on clay minerals and organic matter. 

The overall rate of a fluid-rock reaction can also be modeled, rather than computing the dissolution and precipitation of each solid separately. Eor example, one could write an overall reaction between solids and fluids such as Muscovite -b Quartz = Sillimanite -b K-feldspar -b H2O. The model for overall reactions in metamorphic rocks advanced by Lasaga and Rye (1993)... 

Dissolution and precipitation are chemical reactions by which solids pass into and out of solution. A brief introduction and several examples appear in Chapter 11. These reactions involve equilibria between dissolved species and species in the solid state, and so are described by the general principles of chemical equilibrium in Chapter 14. These reactions rank alongside acid-base reactions in practical importance. The dissolution and reprecipitation of solids permit chemists to isolate single products from reaction mixtures and to purify impure solid samples. Understanding the mechanisms of these reactions helps engineers... 

Type III process consists of the repeated dissolution and precipitation of phase B from the Type II case. It is typically assumed in these calculations that (a) the system is closed to fluid, (b) the stoichiometry of the solid remains constant, (c) the isotopic fractionation factor between the freshly formed portion of the mineral and the fluid is constant (not necessarily equilibrium), and (d) isotopic exchange before dissolution and after precipitation is negligible. As a basis for their modeling, Dubinina and Lakshtanov... 

Reactive transport equations In the formulation proposed by Guimaraes (2002), several dissolved chemical species (solutes) are transported in the water phase through the porous medium and, simultaneously, they react among themselves (homogeneous reactions) and, also, with species present in the solid phase (heterogeneous reactions). The homogeneous reactions include aqueous complex formation, acid/base and oxidation/reduction. The relevant heterogeneous reaction for the case presented in this paper refers to dissolution and precipitation of minerals. 

Numerous processes take place in soil solution, including plant uptake, ion complexation, adsorption and desorption, and precipitation and dissolution (Figure 2.1). As shown in Figure 2.1, Mo solid phases dissolve upon contact with water and provide dissolved Mo in soil solution. The free molybdate ion (Mo04 ) reacts with metals to form complexes and ion pairs in soil solution. Plants absorb dissolved Mo, mainly as Mo04 from soil solution. Removal of Mo04 by plants disrupts the electroneutrality of a soil solution. This causes desorption and adsorption of Mo by oxides, as well as dissolution and precipitation of Mo solid phases in soil solution, until charge balance is reached. The speciation of dissolved Mo in soil solutions must be understood in order to quantitatively describe the availability, toxicity, adsorption, and pre-... 

As a consequence of the concentration gradient in the melt film, a basicity gradient also exists from regions of high p 0 ) at the melt/oxide interface to regions of low p 0 ) at the melt/gas interface. By reaching regions with low p(0 ), the solute experiences a lower solubihty and precipitation of solid oxide occurs by the reversal of the dissolution reactions ... 

Agulyansky et al. [492, 493] investigated the complex structure and composition of solid phases precipitated by ammonia solution from experimental and industrial niobium and tantalum strip solutions. Fig. 136 shows isotherms (20°C) of Nb205 content versus pH for solutions prepared by the dissolution of (NH4)3NbOF6 and (NH4)2NbOF5 in water and of Nb metal in... 

In particular, if we raise the temperature, there will occur some change which will give rise to absorption of heat. If a saturated solution of potassium nitrate is in equilibrium with crystals of solid salt at a particular temperature, and if we now raise the temperature, a change must occur which absorbs heat and so tends to cool the system. This is the dissolution of more solid, because the heat of solution is positive, that is, heat is absorbed when salt goes into solution. But if we have a saturated solution of calcium sulphate, there will occur a precipitation of solid on warming, because the heat of solution is negative, and heat is absorbed when salt comes out of solution. 

In addition to effects on the concentration of anions, the redox potential can affect the oxidation state and solubility of the metal ion directly. The most important examples of this are the dissolution of iron and manganese under reducing conditions. The oxidized forms of these elements (Fe(III) and Mn(IV)) form very insoluble oxides and hydroxides, while the reduced forms (Fe(II) and Mn(II)) are orders of magnitude more soluble (in the absence of S( — II)). The oxidation or reduction of the metals, which can occur fairly rapidly at oxic-anoxic interfaces, has an important "domino" effect on the distribution of many other metals in the system due to the importance of iron and manganese oxides in adsorption reactions. In an interesting example of this, it has been suggested that arsenate accumulates in the upper, oxidized layers of some sediments by diffusion of As(III), Fe(II), and Mn(II) from the deeper, reduced zones. In the aerobic zone, the cations are oxidized by oxygen, and precipitate. The solids can then oxidize, as As(III) to As(V), which is subsequently immobilized by sorption onto other Fe or Mn oxyhydroxide particles (Takamatsu et al, 1985). 

Thus, larger solid/water ratios such as are encountered in pore waters of sediments lead to smaller MgC(>3 contents in the equilibrium magnesian calcites although in either case the magnesium content of the solid increases. Wollast and Reinhard-Derie presented data to support the theory from the standpoint of dissolution and some of our results for the precipitation case... 

The weathering of minerals forms particles with a size continuum from ions to grains. Mineral dissolution and precipitation occur more or less continuously as a function of ambient conditions. Particles of the clay textural fraction may be suspended in solution as colloids as well as occurring as part of the stationary solids. 

In the initial few weeks following submergence, the properties of the soil smface change markedly as a result of reductive dissolution and precipitation reactions. But in time, a steady- or quasi-steady-state is reached, and then the same rules govern the distributions of exchangeable ions between the soil solid and... 

Dissolution and precipitation in the subsurface are controlled by the properties of the solid phases, by the chemistry of infiltrating water, by the presence of a gas phase, and by environmental conditions (e.g., temperature, pressure, microbiological activity). Rainwater, for example, may affect mineral dissolution paths differently than groundwater, due to different solution chemistry. When water comes in contact with a solid surface, a simultaneous process of weathering and dissolution may occur under favorable conditions. Dissolution of a mineral continues until equilibrium concentrations are reached in the solution (between solid and liquid phases) or until all the minerals are consumed. 

Dissolution and precipitation can occur as contaminants travel from the land surface to groundwater aquifers. These processes can affect water chemistry, and they can significantly modify the physical and chemical properties of porous media (Lasaga 1984 Palmer 1996 Dijk and Berkowitz 1998, 2000 Darmody et al. 2000). Under some conditions, large quantities of mass can be transferred between the liquid and solid mineral phases. 

More complex forms of activity term rate laws involve various ions and complexes in solution. Dissolution and precipitation phenomena are in this approach regarded as a summation of individual reactions taking place at the surface of the solid. The net absolute rate is obtained as a summation of individual terms /, each with its specific rate constant k and activity product (Delany et al., 1986) ... 

Secondary phases predicted by thermochemical models may not form in weathered ash materials due to kinetic constraints or non-equilibrium conditions. It is therefore incorrect to assume that equilibrium concentrations of elements predicted by geochemical models always represent maximum leachate concentrations that will be generated from the wastes, as stated by Rai et al. (1987a, b 1988) and often repeated by other authors. In weathering systems, kinetic constraints commonly prevent the precipitation of the most stable solid phase for many elements, leading to increasing concentrations of these elements in natural solutions and precipitation of metastable amorphous phases. Over time, the metastable phases convert to thermodynamically stable phases by a process explained by the Guy-Lussac-Ostwald (GLO) step rule, also known as Ostwald ripening (Steefel Van Cappellen 1990). The importance of time (i.e., kinetics) is often overlooked due to a lack of kinetic data for mineral dissolution/... 

Figure 2 shows the extent of dissolution of red spruce in methylamine, the amount of precipitate collected in the first trap upon complete depressurization to 1 bar, and the Klason lignin content in the wood residue after extraction, as functions of extraction time. The total dissolution and precipitation are normalized with respect to oven dry weight of initial wood. The extraction conditions were 185°C, 275 bar, and 1 g/min solvent flow rate. As shown in the figure, dissolution initially increases with time and levels off at about 28% by weight. The precipitates which were collected as solids follow a similar trend. The Klason lignin content of the wood residue decreases with extraction time, from an initial value of 26.5% down to 10.1% after 5 h of extraction. 

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